The invention relates to an extrusion tool for producing a hard metal rod or ceramic rod having at least one twisted internal borehole, where the outer die ring of the extrusion die exhibits a smooth cylindrical channel.
Hard metal rods or ceramic rods having twisted, i.e. helical, internal boreholes, are further processed, for example, into drills. The twisted internal boreholes form the subsequent flushing and/or cooling channels to feed in the cooling and flushing agent. From the EP 01 81 035 it is known to twist the blank, emerging from the extruder, at an angular speed adjusted to the material flow, the desired drill geometry and to the helical course of the cooling channels by means of suitable twisting devices. To this end, additional twisting devices and adjusted controllers and regulators are required, in addition to the actual extrusion tool. The result of the twisting device attacking from outside the blank emerging from the extrusion tool is the undesired formation of scratches, contact marks and constrictions. An extrusion tool has become known from the DE 36 00 681 A1, where the extrusion compound is already helically twisted during the extrusion process. To this end, the extrusion tool exhibits a die, to whose inner shell is attached at least one flight, which extends helically in the squeezing direction and which forces radially from the outside a twisting motion upon the extrusion compound that is pressed through the die. To form the twisted boreholes, elastic pins are provided that project into the interior of the die and exhibit the desired flushing borehole diameter. No uniform twisting motion, acting over the entire cross section of the blank, can be produced with this extrusion tool, so that the necessary geometry of the . . . the twisted boreholes, elastic pins are provided that project into the interior of the die and exhibit the desired flushing borehole diameter. No uniform twisting motion, acting over the entire cross section of the blank, can be produced with this extrusion tool, so that the necessary geometry of the twisted internal boreholes can hardly be maintained and obtained. Due to the flights arranged on the inner shell of the die, no rod material with a smooth shell surface can be produced; rather shell and outer surface of the produced rod material exhibit pronounced helical impressions. In addition, the twist flights wear rapidly on account of the abrasive behavior of the processed hard metal or ceramic compound, so that the life of the tool is short. The re-conditioning of the dies, e.g. through internal erosion, is expensive and, thus, raises the cost of producing hard metal rods or ceramic rods. Finally, an extrusion tool has already been proposed that exhibits internally a twisting device, which is designed as a twisting screw and with whose aid the a twisting motion is conveyed to the extrusion compound within the extrusion tool during the extrusion process and said compound leaves with twist the outer die ring exhibiting a smooth cylindrical channel. The helical internal boreholes are formed by means of the elastic threads, attached to the twisting device, or by means of the thread-shaped material, emerging from the twisting device and squeezed into the melt stream.
The object of the invention is to further simplify the extrusion tool and to further improve the quality of the rod blanks thus produced.
This problem is solved with a device in that the carrier can be rotated around the longitudinal axis of the die, the flexible elements are formed by threads, and the outer die ring has the shape of a cylindrical ring exhibiting a smooth, cylindrical die channel. The problem is solved with the device in that, instead of the flexible elements forming the internal bore, the carrier exhibits channel openings or outlet openings to corresponding divided circles, from which thready, plastic material can be extruded into the extrusion melt stream. As stated above, the thread carrier and/or the outer die ring can be designed rotatably, i.e. the thread carrier and/or the outer die ring rotate(s) around the longitudinal axis.
Thus, no special twisting device is required that forces a twist upon the entire compound squeezed through the die. In the case of an extrusion tool according to the invention, the extrusion compound does not rotate, while the thread carrier and/or the internally smooth outer die ring is or are set into rotation. Thus, the angle of inclination of the twist channels that are produced is defined by the rotational velocity of the thread carrier and/or the outer die ring and the flow velocity of the extrusion compound. In the case of a rotating smooth die, thus a die or an outer die ring without protrusions, flights or the like, the result of the high extrusion pressure of the squeezing device and the surface friction of the die is that the emerging extrusion compound also rotates with virtually no slippage. In order to rotate the thread carrier a drive is disposed within the extrusion die. The thread carrier is shaped, according to another feature of the invention, as a hub tapering off in the direction of the outer die ring; that is, it exhibits the shape of a propeller hub without blades. Expediently the hub is designed as a hollow hub and exhibits several boreholes, lying on different divided circles, from which the threads are suspended. According to another feature, these threads carry on their ends, projecting into the outer die ring, metallic or other elements influencing the magnetic or electric field. In this manner the rotational velocity of the threads and thus indirectly the twist pitch with known velocity of extrusion of the plastic compound can be measured. Helical channels, which are arranged downstream rotationally symmetrically in the melt stream and which exhibit high precision, are produced by means of the threads of the thread carrier that are arranged at predetermined intervals and in predetermined number.